Like a lot of things in Greek mythology, Tartarus is both a place – the deepest pits of hell – and a deity. One of the primordial entities along with Time, Night and Earth, etc, Tartarus is one of the big guys. He is the third primordial deity, coming after Chaos and Gaia (Earth) and before Eros (love). He may have been the child of Gaia and Aether, who was the primordial deity of upper air (yep that’s a thing).
The pit of Tartarus is so deep it supposedly takes nine days to fall to the bottom from earth, and it’s full of the worst of Greek mythology. King Tantalus, the dude who chopped his child into several pieces and tried to feed him to the gods, is in Tartarus, along with Sisyphus and most of the Titans. However bad the Underworld is, Tartarus is worse. If you take the trip there, you’ll probably meet Hitler, Pol Pot and whoever came up with the Daily Mail’s sidebar of shame.